Semiconductor laser diodes are well-known for their applications in fiber optic communications links. These devices have been used in systems for long distance fiber optic communications since the early 1980's. Laser diodes used in fiber optic communications applications have typically been expensive since sorting of the laser diodes is required to select those devices with desired performance characteristics. Further, packaging and special temperature maintaining circuits contribute to the expense associated with implementation of laser diodes.
Subsequently, laser diodes were developed for use in audio electronics devices such as compact disc players. Laser diodes used in compact disc players are designed to be low cost and have simple packaging requirements as a component of the overall cost objective. Two types of laser diodes were developed for the compact disc player products. The first is a self-pulsating laser diode that has a rather broad spectrum consisting of many longitudinal modes or spectra. Examples of this device are the Sharp LT023, the Mitsubishi ML4014C, and the Sony SLD104AU. These devices were developed to have excellent signal-to-noise (S/N) characteristics even in the occurrence of strong back reflection of light from the application back into the laser cavity. A graph depicting a typical spectral response for the Sharp LT023 device is shown in FIG. 1 and a signal-to-noise curve for the device is shown in FIG. 2. Note that multiple graphs or curves are shown in FIG. 1, each illustrating the response of the device at a specific power output level, i.e. 0.5 milliwatts, 1.0 milliwatts, 3.0 milliwatts and 5.0 milliwatts.
Self-pulsating lasers tend to "ring" strongly with modulation. This characteristic must be filtered out by a fiber optic receiver in order to properly recover the modulated data signal. A self-pulsating laser has been specified in the most widely used low cost fiber optic data communications system standard known as "Fiber Channel". This standard was developed and written primarily by the IBM Corporation participating in the ANSI X3T9.3 Committee. The natural frequency of self-pulsating lasers is typically between 1 and 2 gigahertz (GHz). Thus, the requirement of the Fiber Channel specification that there be at least three self-pulsating periods in a bit cell means that at a bit rate of 266.0 megabits per second (Mb/s), a self-pulsating frequency of at least 1 GHz is desirable. The Fiber Channel specification defines the operating characteristics for fiber optic communications systems operating at 266, 531 and 1062 Mb/s. If self-pulsating lasers are used in the Fiber Channel system that their self-pulsating frequencies would have to be greater than 750 MHz, 1.5 GHz and 3.0 GHz, respectively. The latter two frequencies require selection for the 1.5 GHz specification and may not be possible for the 3.0 GHz frequency. Sorting and selection of laser diodes with these special properties and characteristics dramatically increases the cost of this critical component.
A second type of laser diode known in the industry has a single longitudinal mode response, i.e. it produces coherent light at a single frequency. Examples of this type of component are the Sharp LT022, and the Mitsubishi ML2701, as well as many others known to those skilled in the art. These devices have a spectrum and signal-to-noise characteristics at various power output levels as shown by the curves in FIGS. 3 and 4, respectively. Typically, these devices exhibit a single output frequency as the power supplied to the device is increased. As a result of this characteristic, the single mode laser diodes are more susceptible to light reflected back into the cavity at higher optical output levels, as can be seen in the signal-to-noise curves of FIG. 4. The rise and fall times, i.e. measures of a laser diodes ability to be modulated at high frequencies, are typically very short and in the range of 200-300 picoseconds.
Some manufacturers of laser diodes include "dithering" electronics in the design of the laser diode so that the single mode optical response is broken-up into a series of separate modes. An example of this type of device is the Sharp LT024R10. The inclusion of dithering electronics in the laser diode design adds to the cost and packaging considerations of the device and may not adequately address the overall requirements of a fiber optic data link. For example, a dithering frequency of 300 MHz limits the usable modulation frequency of the laser to frequencies much lower than 300 MHz.
Overall, it is desirable to use single longitudinal mode (single mode) laser diodes because they have optimal beam characteristics and excellent high bandwidth modulation characteristics. On the other hand, single mode laser diodes may present critical problems to multi-mode fiber based communications systems unless the primarily coherent output of the device can be altered so that it behaves more like a self-pulsating laser diode. Thus, it is critical to alter or break-up the coherence of the laser diode optical output to reduce the modal noise associated with single frequency lasers and achieve very high frequency system performance coupled with low bit error rate transmission.
Model noise is an amplitude modulation of the desired optical signal caused by interference effects acting upon the solid-state laser emission. Interference occurs between the various optical modes propagating through an optical fiber, each of the modes being subjected to different delays because of mode dispersion in the fiber. Any change in the output wavelength of the laser, or of the transmission properties of the fiber will tend to alter the interference pattern within the fiber, and accordingly, the mode structure in the fiber. Consequently, an amplitude modulation is produced in the output signal. In fact, it is known that any bending or mechanical repositioning of the fiber alters the mode structure of the fiber. Further, connectors or fiber optic couplers in series in the fiber optic cable also alter the mode structure. When a single mode laser diode supplies an optical signal into a multi-mode fiber, typically several of the propagating modes of the fiber are illuminated. These modes propagate along the fiber with very nearly the same velocity so that over reasonable distances, when one examines the output of the fiber, a "speckle" pattern may be observed. This speckle pattern contains the expected amount of optical power in the aggregate, but in detail the output appears mottled. If a photodiode is used to detect the light carried by the fiber, the speckle pattern may cause a significant variation in the output of the photodiode or detector. This may result in loss of data integrity over the communications link. Typically, the majority of the signal loss occurs at connection joints or at optical couplers in the middle of a fiber optic line where there is a possibility of creating noise in the optical fiber. This is a result of misalignments of the fibers at the coupling or connector interface. If the cores of the fibers, which are typically 0.002 inches in diameter, are not perfectly aligned the light becomes "unguided" and is lost to the system. Usually, a loss at a fiber connector is not terribly detrimental, but when there is a speckle pattern present (attributable to the single frequency laser diode) at the coupler/connector interface, the amount of loss can become randomly time varying. A time variation in optical signals carried by multi-mode optical fibers is typically not synchronous with any system time constant so it results in noise commonly known as modal noise.
A simplified method and apparatus for a fiber optic transmitter that incorporates a single longitudinal mode laser diode in an optical communications system would simplify and reduce the cost of fiber optic communication links, yet enable increased data throughput rates. An economical technique for obtaining multi-mode output from a single mode laser diode is needed to take advantage of the desirable characteristics of such laser diodes.